High Force, the UK's largest waterfall at 21 metres high is located in the upper course. An area of hard rock, called Whin Sill (or Whinstone), is located above a layer of soft rocks (sandstone and shale) and together they create this impressive waterfall.
High Force waterfall
As the River Tees starts to erode sideways (lateral erosion), it forms meanders. These can be identified in the middle course near Barnard Castle.